Well screens are available that consist of a helically coiled stainless steel wire of trapezoidal cross section, welded to an inner cage of axially extending stainless steel wires. Well screens of this type are referred to as wedge wire screens. There is a narrow gap between successive coils of the coiled wire, forming a helically extending slot through which water can, in use, enter from a surrounding aquifer into the well screen. As a result of the trapezoidal cross-section of the coiled wire, the slot widens from the outside of the well screen inwardly. The advantages of tapered slots, i.e. slots that widen from the outside of the screen inwardly, are well known. Wedge wire screens are, however, very expensive. Furthermore, although stainless steel does not corrode easily, corrosion cannot be eliminated entirely. Corrosion causes encrustation, which has the effect of reducing the slot area. Corrosion also weakens the structure of the well screen.
Well screens that consist of extruded plastic pipes are also available. In this type of well screen slots for the ingress of water are formed by producing cuts in the wall of the pipe. Cut slots have parallel slot walls so that well screens of this type do not have the advantages offered by well screens with tapered slots. A major disadvantage of slots having parallel slot walls is that they are far more prone to clogging than is the case with tapered slots. Moreover, the cutting process tends to produce slots with rough slot walls, and this aggravates the clogging problem in that soil particles more readily become lodged in slots which have rough slot walls. In one form of extruded well screen the slots extend across the pipe, i.e. at right angles to the longitudinal direction of the pipe. Slots of this configuration significantly reduce the ability of the pipe to withstand axial loads.
In applicant's U.S. Pat. No. 4,626,129 there is disclosed an apertured pipe segment which has tapered slots, the configuration of the pipe segment being such that it can easily be made from a plastics material by means of an injection moulding process. The pipe segments are formed with a socket at one end, into which the other end of an adjacent segment is a close fit. This requires the pipe segment to have an external collar at the socket end. If a number of such pipe segments are connected end-to-end, the assembly can be used as a well screen. It has been found, however, that the collars on such an assembly lead to snagging when the assembly is lowered into or extracted from a borehole.
It is an object of the present invention to provide an apertured pipe segment which has tapered slots and can easily be made by means of an injection moulding process, yet does not have an external collar to cause snagging in a borehole. It is a further object of the invention to provide an apertured pipe segment with slots which are better able to prevent soil particles from entering through the slots into the pipe segment, yet permit water to enter. It is still a further object of the invention to provide a well screen which is sufficiently strong to withstand the forces encountered at the bottom of a borehole. Further objects of the invention will become apparent from the description hereinafter of the preferred embodiments.
According to the invention there is provided an apertured pipe segment having a pipe wall and apertures in the pipe wall for the ingress of water into the pipe segment, the apertures each being formed by a recess in the outside of the pipe wall and an inner slot defined by opposed slot walls which diverge from the bottom of the recess inwardly to the inside of the pipe segment, the recess being in flow communication with the inner slot via a slot mouth at the bottom of the recess, which slot mouth is narrower than the smallest transverse dimension of the recess.
The inner slots may extend longitudinally of the pipe segment. In other words, where, as would be convenient, the pipe segment is of round cross section, the inner slots would extend in the axial direction.
Each of the inner slots may, across its entire width and depth, be open longitudinally. If the pipe segment is made by injection moulding, this configuration of the inner slots will permit them to be formed by fins on a mould part which can be withdrawn axially during demoulding.
Said recesses may be in the form of slots (referred to herein as the outer slots), defined by opposed slot walls, the outer slots being aligned with the inner slots.
The slot walls of each of the outer slots may diverge from the outside of the pipe segment inwardly.
Each of the outer slots may, across its entire width and depth, be open longitudinally.
The inner slots may be open longitudinally in the direction of one end of the pipe segment, and the outer slots may be open longitudinally in the direction of the other end of the pipe segment.
The pipe segment may have a socket formation at said one end and a spigot formation at said other end, the spigot formation being complementary to the socket formation, so that a series of such pipe segments can be interconnected end-to-end with the spigot formation of one pipe segment entering into the socket formation of the next.
Further according to the invention there is provided an apertured pipe which comprises a series of pipe segments each having a socket formation at one end thereof and a complementary spigot formation at the other end thereof and the pipe segments being connected together end-to-end with the spigot formation of one pipe segment being accommodated in the socket formation of the adjacent pipe segment, each pipe segment having a pipe wall and apertures in the pipe wall for the ingress of water into the pipe, each aperture being formed by an outer slot which extends from the spigot formation in the direction of the socket formation and an inner slot which extends from the socket formation in the direction of the spigot formation, the outer slot being in flow communication with the inner slot via a slot mouth at the bottom of the outer slot, which slot mouth is narrower than the smallest transverse dimension of the outer slot, and the inner slot being defined by opposed slot walls which diverge from the slot mouth inwardly to the inside of the pipe segment.